This invention relates generally to the manufacture of integrated circuits, and more particularly to reticle boxes which hold the reticles used in optical stepper machines to project an image of an integrated circuit layer onto a semiconductor wafer.
Photolithography is a necessary process in the production of most integrated circuits. The photolithography process includes the steps of forming a resist layer over semiconductor wafer, exposing the resist layer to a patterned light source, developing the resist layer, processing the semiconductor wafer through the developed resist layer, and removing the resist layer.
An optical stepper apparatus is often used to expose the resist layer. An image of a layer of an integrated circuit die is formed on a small, square piece of glass called a reticle. The reticle is placed in the optical stepper apparatus and the image of the layer is reduced and projected onto a portion of a resist layer covering the semiconductor wafer. The reticle is then indexed and the reduced image is focused on another portion of the resist layer and so forth until a majority of the resist layer has been exposed. A typical reticle is about 5-7 inches square, and the image of the layer is typically reduced to about 300-500 millimeters square.
Reticles are often stored in reticle boxes. Some optical stepper apparatus include a stepper reticle loader mechanism which automatically opens a reticle box, inserts the reticle into the stepper apparatus, exposes the wafer, returns the reticle to the stepper box, and closes the box. This type of automated handling is preferable because it minimizes contamination of and damage to the reticle and the stepper apparatus.
FIGS. 1 and 2 illustrate a reticle box which is commercially available from Canon, Inc. of Tokyo, Japan. FIG. 1 is a top plan view of a closed reticle box 10, and FIG. 2 is a cross-sectional view of the box taken along line 2--2 of FIG. 1. With reference to both figures, the reticle box 10 includes a base member 12 and a cover member 14 which is slidingly engageable with the base member 12. A camming bar 16 locks the cover member 14 to the base member 12. When closed and locked, the reticle box 10 surrounds a reticle 18 disposed within the box. The reticle is supported on its bottom surface by four base stand-offs 20 which include a resilient portion 22. Four cover stand-offs 24 extend close to, but do not touch, a top surface of the reticle 18.
In FIG. 1, it can be seen that the cover member 14 is an assembly of several parts including a frame portion 26 and a central cover portion 28. It is the frame portion 26 of the cover member 14 which engages with walls 30 of the base member 12. A pair of metal hinge pins 32 connect the frame portion 26 to the central cover portion 28. The hinge pins permit the opposing end of the central cover portion 28 (i.e. the end near camming bar 16) to pivot slightly upwardly and downwardly. A pair of metal leaf-springs 34 are attached to frame portion 26 and bias the pivoting end of the central cover portion away from the base member 12.
When the base member 12 and cover member 14 are fully engaged, the camming bar 16 may be rotated to force down the pivoting end of the central cover portion 28. This causes a pair of tabs 36 to engage with slots (not seen) on the upper portion of base member 12 to lock the cover member 14 to the base member 12. Even in this locked position, the reticle 18 is free to move slightly from side-to-side (as indicated by arrow 38 in FIG. 2), or up-and-down (as indicated by arrows 40 in FIG. 2).
Reticle box 10 has several drawbacks. For one, it is made from a plastic material which can accumulate a considerable amount of static charge--sometimes in excess of 1000 volts d.c. This high static charge can disrupt or damage delicate circuitry, mechanisms and optics of an optical stepper apparatus.
Another serious problem is the possibility of damage to a reticle 18 due to the loose way it is held within the reticle box 10. As the reticle moves back-and-forth and up-and-down, it can become scratched or broken. Since the reticle 18 sits loosely on the base stand-offs 20, the reticle box 10 must be stored in a flat, upright position. This means that a number of reticle boxes 10 cannot be compactly stored on their sides, nor can the reticle box 10 be used for shipping reticles.
Another problem with reticle box 10 is the use of metal hinge pins 32 and leaf springs 34. These metal surfaces rub against plastic surfaces of the cover member 14, creating a shower of particles which can contaminate the reticle box 10 and/or the reticle 18. With today's microscopic feature widths in I.C. devices, even small amounts of contaminants can damage an IC during the production process.
The reticle box 10 also has problems with contamination from yet another source. The walls 30 of base member 12 have peripheral flanges 42 which press against peripheral flanges 44 of cover member 14. Even when camming bar 16 is in a locked position, openings around the perimeter of the reticle box 10 allows a free path for particle entry into the box past these flanges as indicated by arrows 46.
Another drawback of reticle box 10 is that it has many parts and is therefore relatively difficult and expensive to manufacture. For example, the cover member 14 is a multi-part unit including a number of pins, leaf-springs, screws, etc. It would, of course, be desirable to reduce the number of parts of the reticle box to reduce its cost and to increase its reliability.